This invention relates to a document feeding mechanism which may be a part of an automatic line finding device of a business machine like an accounting machine.
The prior art document feeding mechanisms which are used with automatic line finding devices generally use opposed feed rollers which grip the document to be fed therebetween, and one of the rollers is driven or rotated while the other one is simply rotated in conjunction therewith and presses the document against the driven roller. This type of feeding arrangement has the disadvantage of introducing inaccuracies into the system as the diameters of the feed rollers begin to get smaller due to wear. It is obvious that as the diameters of the feed rollers get smaller due to wear or due to a variation in feed rollers having diameters which are less than the desired nominal diameter, each rotation of the fed rollers will not produce the desired linear displacement of the record medium or document being fed thereby, resulting in inaccuracies in locating a particular line of the document at the print station in the business machine.
The present invention obviates the problems mentioned in the previous paragraph in that the document feeding mechanism disclosed herein utilizes a gear driven endless belt and counter rollers which maintain the document in feeding engagement with the endless belt. Because of the gear driven belt, there is no slippage between the endless belt and its driving means, and the use of a gear driven belt greatly reduces errors due to manufacturing tolerances when compared to feeding arrangements using opposed rollers. For example, if a driving gear has 10 teeth (n.sub.1) and the endless belt has 100 complementary teeth (n.sub.2) and a belt length (L) of 500 mm., each complete rotation of the drive gear will move the belt a distance (s) where: EQU s = (n.sub.1 L/n.sub.2) = 10 (500)/100 = 50 mm.
A typical endless belt having a nominal length of 500 mm. may have a length tolerance of .+-. 0.16 mm. Thus, after one rotation of the driving gear, the maximum displacement error of the endless belt is .+-.0.016. If a conventional feed roller is used for feeding a document the same distance (50 mm.) in one complete rotation of the feed roller, the feed roller would have to have a nominal diameter of approximately 15.9 mm. A typical tolerance for a feed roller having a nominal diameter of 15.9 mm. is .+-. 0.05 mm.; and consequently, the maximum error after one complete revolution of the roller is (77 .times. 0.05) or about 0.15 mm. This maximum error of 0.15 mm., for a conventional, opposed, feed roller arrangement, is about ten times the error which is introduced by the document feeding mechanism of this invention.
Another feature of this invention is that the document feeding means disclosed herein utilizes a novel document engaging structure which is used in conjunction with the endless belt to provide a simple, and economical means for feeding documents for use with an automatic line findng device.
Some prior art feeding devices are shown in British Pat. No. 1,349,295 which waas published on Apr. 3, 1974, and U.S. Pat. No. 3,894,624 which issued on July 15, 1975.